1. Field of the Invention
The invention relates to an image reading system for reading an original image in an image scanner, a facsimile apparatus, or the like and, more particularly, the invention is suitable for use in an image reading system with a construction such that an original is illuminated by an irradiation light from an LED (light emitting diode) and an image of the original is photographed by a CCD (charge coupled device) one-dimensional image sensor.
2. Related Background Art
In a conventional image scanner or a facsimile, as an image reading system for reading an image on an original, there is a system as shown in a cross sectional view of FIG. 13. In FIG. 13, a sensor IC 1 is installed on a sensor board 2 and is positioned by a frame 6 together with a lens array 3, an LED array 18 serving as a light source to illuminate an original, and a cover glass 5.
The LED array 18 is constructed by arranging a plurality of red (R) LEDs 13, green (G) LEDs 14, and blue (B) LEDs 15 in a line onto an LED board 7 so as to have predetermined intervals as shown in FIG. 14.
Since it is necessary to uniformly irradiate the light over a whole width of an original to be read, the LED array 18 serving as a light source is constructed by at least about 30 LEDs with respect to each color of R, G, and B in order to irradiate the original of, for example, the A4 size.
In the conventional image reading system as mentioned above, however, in case of installing a number of LED devices of each color of R, G, and B onto the LED board, since the LED device is very expensive, an LED array using tens of LEDs becomes a largest cause of an increase in costs in the whole system.
According to an illuminance distribution on the original surface illuminated by the LED array, since a portion corresponding to the top of the LED is bright and a portion between the LEDs is dark, peaks of only the number of LEDs appear. Therefore, when the original of a uniform density is read, an output between the adjacent sensor devices are steep and it is necessary to correct the sensor output with respect to all of the bits of the sensor device. Particularly, in case of a color image sensor, a correcting circuit to correct each sensor output is needed with regard to each of three colors of R, G, and B. Not only a circuit construction is complicated but also it becomes a cause of a further increase in costs.